One widespread problem with ceramic materials, which are employed, for example, in PTC components, is the profile of the resistance/temperature characteristic curve while complying with required material parameters. PTC components are temperature-dependent resistor components (thermistors) having a positive temperature coefficient (PTC=positive temperature coefficient). Their resistance increases with increasing temperature.
For heater applications by way of example, for example, auxiliary dielectric heaters of motor vehicles, a high power consumption is required with the lowest possible operating temperature of the individual components, as well as very good control properties of these heating elements. At the same time, the breakdown voltages (UD) and the rated resistance (RR) must correspond to requirements. The breakdown voltage (UD) is the highest voltage which the component can withstand. At a voltage above this value, the component loses its functional property and may possibly be destroyed. The rated resistance (RR) is the resistance value of the unloaded component at a particular rated temperature (TR). The generally conventional rated temperature is 25° C.
For motor start-up applications, for example, in refrigerator compressors, in order to improve the energy efficiency it is desirable to reduce the residual power at the working point while maintaining sufficient switch-on times and switch-on currents. At the same time, however, the high breakdown voltage (UD) and the short cooling times of the component must be preserved for reliable operation. In this way, inter alia, a substantial energy savings can be achieved.
For application for example in the field of overload protection, a control ratio as high as possible between the rated current and the residual current at working points is desired, with the lowest possible operating temperatures. In this way, substantially higher security can be achieved in the event of overload.
For example, in sensor applications for determining temperatures using PTC components, a relatively wide temperature window for the signal temperature is specified on the basis of the characteristic curve shape, which restricts the application of PTC temperatures. A steep rise from the minimum temperature (TMIN) to the maximum temperature (TMAX) would permit reduction of the temperature window and therefore more precise adjustment of the switching temperature. The minimum temperature (TMIN) is in this case the temperature at which the minimum resistance (RMIN) is reached. Similarly, the maximum temperature (TMAX) is the temperature at which the highest resistance in the measurement range, i.e., the maximum resistance (RMAX), is measured.
The following list of reference symbols may be used in conjunction with the drawings:    1 characteristic curve of composition 1 not according to the invention    2 characteristic curve of composition 2 according to the invention    3 characteristic curve of composition 3 according to the invention    4 characteristic curve of composition 4 according to the invention    5 characteristic curve of composition 5 according to the invention